Modulating system



y 9 V J. c FERGUSON ETAL 7 3 IODULA'HNG SYSTEM Filed NOV. 1, 1944 3 Sheets-Sheet 1 FIG.|

FIG.3

\UTPUT VOLTAGE/INPUT vou'.

u 1 l E a 5 F3 F F0 F2 rasouencv 3 O 0- POWER A!!! l5 ANT.

FIG. 5

- nousounc:

' 4| I ANT.

INVENTORS 4 POWER I JOSEPH C. FERGUSON 2 T T. MAYLE,JR.

May 10, 1949. J. c. FERGUSON zrm.

I00. SOURCE Sheets-Sheet 2 ANTENNA POWER AIP.

INVENTORS JOSEPH C. FERGUSON ALBERT T. MAYLE, JR

ATTO R N EY y 1949- J. C. FERGUSON mm. 2.470,

NODULATING SYSTEM Filed Nov. 1., 1944 3 Sheets-Sheet 3 ANTENNA 1 ANTENNA as 2 g I POWER All? 71' as s4 POWER AMP.

l l l l l FIG.8

FIG.7

INVENTORS JOSEPH. G FERGUSON ALBERT T. MAYLE, JR.

ATTORNEY Patented May 10, 1949 MODULATIN G SYSTEM Joseph C. Ferguson and Albert T. Mayle, In, Fort Wayne, Ind., assignors, by mesne assignments, to Farnsworth Research Corporation, a corporation of lndiana Application November 1, 1944, Serial No. 561,354

6 Claims.

This invention relates to signal-modulating systems and more particularly to a novel method and system for wide-band amplitude modulation ol :1 carrier signal.

Modulation systems of this type are used for obtaining amplitude-modulated carrier signals [or 'radio and television transmission. Widebaud modulation is essential for the faithful reproduction of the picture signals in high definition television systems. A conventional amplitudv modulating system operates on the principle of absorbing energy from the carrier signal output in order to obtain modulation thereof. This is riicctcd by providing controllable variable resistors for bypassing a larger or smaller portion of the carrier energy developed by a power amplifier. Space discharge tubes are provided which act as variable resistors, the value of which is varied in accordance with a modulating signal. However, a system of this type has the drawback that the resistance of space discharge tubes can only be varied within certain limits. Although the resistance of a discharge tube can be made to approach infinity, its value can not be decreased below a certain limit which is of the order of magnitude of several hundred ohms. It has also been suggested to invert the impedance represented by the resistance of the discharge tubes by a suitable transmission line. Even with this modification, however, the resistance can only be varied within certain narrow limits which imposes a definite limitation on the percentage modulation obtainable with this system.

It is an object of the present invention, therefore, to provide a novel system for obtaining wideband amplitude modulation of a carrier signal.

.Another object of this invention is to provide a signal-modulating system for obtaining a high percentage modulation of an ultra-high carrier frequency.

A further object of this invention is to provide a network including a capacitance, the value of which may be varied over a wide range at high or low frequency rates in response to an electrical modulating signal.

In accordance with the pres nt invention there is provided a signal modulating system comprising an input circuit and an output circuit. The input circuit is coupled to the output circuit by means including a variable capacitance element. Electronic means are provided for varying the capacitance element in accordance with a modulating signal applied to the input circuit. In this manner an amplitude-modulated carrier signal is developed in the output circuit which is then utilized, for instance, in a transmitting antenna.

In accordance with an embodiment of the invention the variable capacitance element provided for coupling the input circuit to the output circuit includes two spaced electrodes. Means are provided for introducing free electrons be tween the electrodes, and the capacitance of the element is varied by influencing the free electrons in accordance with a modulating signal. In accordance with the present invention this may be accomplished by developing an electron beam and passing it between the electrodes of the capacitance-element. This electron beam may be deflected, or the density of the electron beam may be varied, or the width of the electron beam may be varied all in accordance with a modulating signal.

According to another embodiment of the invention an electron cloud is created between the electrodes of the variable capacitance clement. Means are provided for controlling the density of the electron cloud in accordance with a modulating signal.

It will be seen that in each of the foregoing embodiments of the invention free electrons are.

introduced between the electrodes of a capacitance element. The density or the relative position of the electrons with respect to the electrodes is varied in accordance with a modulating signal. In this manner the capacitance of the electrodes can be varied within comparatively wide limits at high and low frequency rates.

For a better understanding of the invention, together with other and further objects thereof, reference is made to the following description, taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

In the accompanying drawings: 4

Fig, 1 is a circuit diagram of a modulating system embodying the present invention;

Fig. 2 is a set of curves referred to in explaining the operation of the invention;

Fig. 3 is a circuit diagram of another modulattionship between the ratio Fig. 4 is a circuit diagram of a modulating system including a tuned circuit arranged for absorbing energy;

Fig. 5 is a circuit diagram of a modulating system employing an electron discharge tube for coupling a carrier signal generator to a transmitting antenna; v

Fig. 6 is a circuit diagram of a modulating system including a cathode ray tube arranged .for controlling the intensity of the electron beam thereof;

Fig. 7 is a circuit diagram of .a modulating system including a cathode ray tube arranged for controllably deflecting the electron beam; and

Fig. 8 is a circuit diagram of a modulating system including a cathode ray tube wherein means are provided for controlling the width of the electron beam.

Referring now more particularly to Fig. l of the drawings, there is shown a modulating system including a high frequency carrier generator and amplifier for developing a carrier signal. Any conventional carrier signal generator may be used for this purpose such as a power amplifier. The

carrier signal developed by generator and ampliher I is impressed upon coil 2 of input circuit 3. Input circuit 3 is coupled to output circuit 4 through tuned circuit 5. Tuned circuit 5 includes coil '6 inductively coupled to coil 2 of input circuit 3 and coil 1. Coil 1 is inductively coupled to coil 8 of output circuit 4. Tuned circuit 5' is tuned by variable condenser III. Output circuit 4 is connected to transmitting antenna II.

In accordance with the present invention amplitude modulation of the carrier signal developed by generator I is efiected by controllably detuning circuit 5 which couples input circuit 3 to output circuit 4. Circuit 5 is detuned by varying the capacitance of condenser I6 electronically, in a manner to be described hereinafter.

Fig. 2 shows a set of curves illustrating the relaof the output voltage present inoutput circuit 4 divided by the input voltage present in input circuit 3 and the frequency of a tuned circuit of the type shown in Fig,

1. Curve I2 shown in Fig. 2 represents theoutput voltage for a constant input voltage plotted against frequency of circuit 5 tuned to a certain frequency in. When tuning condenser I0 is aried so that the resonant frequency of the rircuit is now ii, the relationship between the output voltage for a constant input voltage and frequency is represented by curve I3. As is well known the resonant frequency of tuned circuit 5 decreases when the capacitance of condenser I0 increases. Similarly, by decreasing the capacitance of condenser I 0 the resonant frequency of the tuned circuit can be increased to a higher value j: and now the relationship between output voltage and frequency is represented by curve I4.

We may now assume that the frequency of the carrier signal developed by generator I is 13 shown in Fig. 2. Preferably the carrier frequency is is difl'erent from the resonant frequency )0 of tuned circuit 5 when condenser III is adjusted to its normal value. It will be evident that by varying the capacitance of condenser I0 about its normal value, which makes tuned circuit 5 resonant at the frequency In, the impedance of tuned circuit 5 is changed. Hence, tuned circuit 5 transmits a larger or smaller amount of carrier able condenser 2| and coils signal energy from generator I to output circuit 4 and, hence, to transmitting antenna II.

In accordance with the invention the input circult may also be coupled to the output circuit or the modulating system by means of a tuned shunt. Such an arrangement is illustrated In Fig. 3. The carrier signal generator and amplifier is connected to input terminals I5, I6, while the useful load or antenna is connected to output terminals I1, I8. Tuned shunt 2|) shunts input terminals I5, I6. Tuned shunt 20 includes vari- 22 and 23 arranged symmetrically about the condenser 2|. Tuned shunt 20 can be detuned .by varying condenser 2|, Preferably, the normal value of condenser 2| is such that shunt 20 is not exactly tuned to the frequency of the carrier signal. Leads I5 and I6 should each contain a Series impedance which, however, will always be present in the carrier signal generator. Therefore, no series impedances have been shown in Fig. 3. By increasing or decreasing the capacitance of tuning condenser 2| a larger or smaller amount of energy of the carrier signal is shunted, and thus the carrier signal appearing across output terminals I1, I8 is amplitude-modulated. The capacitance of, tuning condenser 2| may be varied electronically.

Still another modulating system in accordance with the invention is illustrated in Fig. 4. Here a power amplifier or carrier signal generator is connected to input terminals I5, I6. Input terminals I5, I6 are connected to output terminals I1, I8 through coils 25, 26, respectively. A tuned resonant circuit 21 includes coils 28 and 30, tuning condenser 3| and resistor 32. Coil 28 is inductively coupled to coil 25 connecting terminals I5 and I1. Similarly, coil 36 is inductively coupled to coil 26 connecting terminals I6 and I8. The resonant frequency of tuned circuit 21 can be varied by changing the capacitance of tuning condenser 3|. Thus, a larger or smaller amount of power can be absorbed by tuned circuit 21, and this power is dissipated in load resistor 32. Hence, a modulated carrier signal is developed across output terminals I 1, I8 which may be con,- nected, for instance, to a transmitting antenna.

In accordance with the invention various means may be utilized for electronically varying the capacitance of the tuning condensers shown in the modulating systems of Figs. 1, 3 and 4. Such means are illustrated in Figs. 5, 6 and 7. The embodiment of the invention illustrated in Fig. 5 utilizes a virtual cathode which is created in an electron discharge tube. This virtual cathode is controlled in accordance with a modulating signal for varying the capacitance between electrodes or grids of the tube. It is well known that in some types of high vacuum electron discharge tubes, including triodes, tetrodes, pentodes and hexodes an electron cloud can be created between certain electrodes or grids thereof. An electron cloud of this type has sometimes been referred to as a virtual cathode. The name has originated because this electron cloud may serve as a source of electrons and thus acts as a cathode. By changing the potentials applied to the electrodes of the tube the density of this electron cloud can be varied. In accordance with the invention this variable electrode capacitance is utilized for coupling a high frequency carrier signal generator to a useful load, such as an antenna.

As shown in Fig. 5 there is provided an electron discharge tube 35 of the tetrode type. Tetrode 35 has a cathode 36, control grid 31, screen grid 38 and anode 46. Cathode 36 is connected to ground, as shown. Control grid 31 is supplied with a positive potential from potentiometer 4| connected to battery 42 having its negative terminal grounded, as shown. Screen grid 38 is supplied with negative potential through potentiometer 43 connected to battery 44 having its positive terminal grounded, as shown. High frequency choke 45 is connected between potentiometer 43 and screen grid 38. Anode 40 is connected to a source of positive potential +B.

' An electron cloud is formed in tube 35 between control grid 31 and screen grid 38. The density of this electron cloud can be varied by varying the potential supplied to control grid 31. In accordance with the invention a modulating source is connected to coil 46 inductively coupled to coil 41 which is arranged between potentiometer 4| and control grid 31. The varying potentials created by the modulating source will change the control potential applied to control grid 31. Hence, the density of the electron cloud arranged between control grid 31 and screen grid 38 varies in accordance with the modulating source. It follows from the previous explanation that the value of the capacitance between grids 31 and 38 changes in accordance with the density of the electron cloud therebetween, that is, in accordance with the modulating source.

This variable capacitance may be utilized in the tuned shunt illustrated in Fig. 3. To this end coils 23 and 22 are connected, respectively, to grids 31 and 38 through blocking condensers 52 and 53. Blocking condensers 52 and 53 serve for separating the direct current potentials supplied to grids 31 and 38 from tuned shunt 54. The capacitances of condensers 52 and 53 should be relatively large in comparison with the capacitance of electrodes 31, 38. Coil 22 is connected between input terminal l and output terminal l1 and coil 23 is connected between input terminal |6 and output terminal l8. A power amplifler or ultra-high frequency carrier generator may be connected to input terminals l5, l6, while output terminals I1, l8 may be connected to a useful load, such as a transmitting antenna.

The operation of the circuit illustrated in Fig. 5 will now be evident. The modulating source connected to coil 46 impresses voltage variations upon coil 41 and thus modulates the control po- V .tentials applied to control grid 31. Consequently, the density of the electron cloud between grids 31 and 38 is varied which in turn changes the capacitance of the two grids. Variations of the capacitance of grids 31 and 38 changes the resonant frequency of tuned shunt 54 and, therefore, the impedance of shunt 54 is varied in accordance with the modulating source. In this way a larger or smaller amount of power is bypassed by shunt 54 and, therefore, the carrier signal developed across output terminals |1, I8 is amplitude modulated.

Instead of a tetrode as illustrated in Fig. 5, a triode or a pentode or other multi-grid electron discharge tube may be used for creating an electron cloud and varying the density thereof.

In accordance with the present invention the value of a capacitance may also be varied electronicallyby means of an electron beam. The embodiment of the invention illustrated in Fig. 6 shows means for varying the intensity of an electron beam. In Fig. 6 there is shown a cathode ray tube comprising an evacuated envelope 55. An electron beam is developed and focused by cathode 56, control grid 51 and first anode 58. By

6 means of potentiometer 68 connected to battery 6| anode 58 is maintained at a positive potential with respect to cathode 56, and control grid 51 at a negative potential with respect to cathode 56.

A modulating source is connected to coil 62 inductively coupled to coil 63. Coil 63 is arranged between potentiometer 68 and control grid 51. Voltage variations developed across coil 62 are impressed upon coil 63 and thus vary the potential applied to control grid 51. This in turn varies the density or the intensity of the developed electron beam. The electron beam is collected by collector anode 64 which is held at the same potential as first anode 58. Electrodes 65, 65 are spaced symmetrically about the electron beam indicated at 66. Electrodes 65, 65 are connected in a resonant circuit including coils 6 and 1. Coil 6 is inductively coupled to coil 2 connected to a power amplifier and coil 1 is inductively coupled to coil 8 which may be connected to a transmitting antenna. Carrier frequency choke 61 is connected between coils 6 and 1 and collector anode 64, and blocks oscillations at carrier frequency from collector anode 64, while passing the direct current supplied from battery 6| through potentiometer 68 so that electrodes 65 are maintained at the same potential as anodes 64 and 58.

Electrodes 65 and coils 6, 1 represent a tuned circuit, such as circuit 5 shown in Fig. 1. The capacitance of electrodes 65 can be varied by varying the intensity or density of the electron beam 66 as explained previously. This is accomplished by varying the potential applied to control grid 51. Hence, the capacitance of electrodes 65 is varied in accordance with the modulating source, and an amplitude-modulated carrier signal is impressed upon coil 8.

Instead of varying the density of the electron beam the electron beam may be deflected to vary the capacitance of electrodes arranged unsymmetrically about the electron beam. Such an arrangement is shown in Fig. '1. The modulating circuit shown here includes a cathode ray tube having a cathode 56, control grid 51 and first anode 58 which are supplied with suitable potentials from potentiometer connected to battery 6|.

Electron beam 66 is collected by collector anode 64 which is maintained at the same potential as first anode 58. Defiecting plates 18, 10 are provided for deflecting the electron beam. Deflecting plates 10 are connected to a modulating source which is shunted by coil 1|. Audio frequency choke 12 is connected between the center tap of coil 1| and lead 13 connecting potentiometer 60 to first anode 58. Hence, deflecting plates 18 are held at the same potential as first anode 58 and collector anode 64.

Electrodes 65, are arranged unsymmetrically about electron beam 66. In other words, electron beam 66 in its undefiected position is nearer one electrode than the other one. Electron beam 66 is deflected by deflecting plates 10 in accordance with voltage variations generated by the modulatin source. By deflecting electron beam 66 the capacitance of electrodes 65 is varied in accordance with the modulating source. In a similar manner as illustrated in Fig. 6 electrodes 65 are connected in a tuned circuit includingcoils 6 and 1 inductively coupled to input coil 2 and output coil 8, respectively. Carrier frequency choke 61 is arranged between coils 6 and 1 and collector anode 64. Otherwise, the circuit operates in the same manner as explained in connection with Fig. 6.

In accordance with the present invention the value of a capacitance may also be changed by vary the capacitance of said electrodes. thereby varying the width of an electron beam. As to develop an amplitude-modulated carrier signal shown in Fig. 8, there is provided a cathode ray in said output circuit.

tube 55 including cathode 56, control grid 51 A si nal modulating system compr si an and firstanode 58 which are maintained at suit- 5 input circuit and a u p t c means for pable potentials by potentiometer 60 connected to p y a c r S g a o a d input circuit, a battery 6|. The electron beam is collected by tuned Circuit for Coupling a d pu c cu t t collector anode 64 In order t vary t width said output circuit, said tuned circuit including a of the electron beam there is provided a focusing Variable u ped capacitance element comprising ring 15. Focusing rin 15 is connected to coil 63 10 o Spaced electrodes, means for developing an coupled to coil 62 which in turn is connected to electron beam and for passing it through e a modulating source. Coil 63 and focusing ring Space between S odes i ut mp c are connected to potentiometer 60 and are maino electrodes, and means for Varying e elatained at a potential between that of anode 53 tlve position of said electron beam with regard and collector anode 54 on the one hand and 15 to said electrodes in accordance with a modulatcathode 56 on the other hand. The potential s signal to vary e apacitance of said elecwhich issupplied to focusing ring 15 is modified trodes, thereby to develop an amplitude-moduor varied by the modulating source through coils latod Carrie! Signal i d output circuit.

62 and 63. Thus the focusing of the electron A e s modulating s stem c mp ising an beam is changed For a certain potential on 1nput.c1rcuit and an Output circuit, means for focusing ring 15 electron beam 16 is focused on applying a carrier signal to said inp circut, collector anode 64 while for another potential means including a Variable element av only applied to ring 15 the electron beam represented lumped capacitive reactance for coupling said by lines 11 would be fo u ed behind ooueotor input circuit to said output circuit, said variable anode 54 Hence the width of the electron e element comprising two spaced electrodes, means is varied between electrodes 65 which are disfor developing an l on b am a d f r p s in posed symmetrically about t electron beam. it through the space between said electrodes with- Accordingly, the capacitance of electrodes 65 is out Impacting Said l d s, and means for varied in accordance with the modulating source. Varying the Width of sa d electron beam in ac- In the same manner as described in connecoordanoo With a modulating Signal t V y e tion with Fig 6 electrodes 55 are connected to capacitance of said electrodes, thereby to develop a tuned circuit including coils s and 1 inductively an d du at d ar i r s nal in said coupled to coils 2 and 8, respectively. Carrier output clroultfrequency choke 61 is arranged between coils 6 A signal modulating System comprising an and l and collector anode 64. The modulating to input Circuit and an ut circuit, means for circuit of Fig. 8 operates substantially in the applying a Carrier si nal to said input circuit, a same manner as the one illustrated in Fig. 6 tuned circuit for coupling S d put Circuit to t the exception t the capacitance of e1eo said output circuit, said tuned circuit including trodes 65 is varied by varying the width of the e variable lumped c p ci ance element compriseleotron heam 4 mg two spaced electrodes, means for developing Throughout t figures like components have an electron beam and for passing it through the been designated by the some r f re numerals space between said electrodes without impacting It is to be understood that an electronically said electrodes, and means for i g he width variable capacitance obtained according to any of electron beam accordance a moduof the embodiments of the invention shown in latlng Signal to Vary t paci ance of said elec- Fig 5 6, 7 and 3 may be used with any f t trodes, thereby to develop an amplitude-modusignal modulating circuits illustrated in Figs. 1, lated F Signal in S id Output circuit.

3 and 4, Instead of the signal modulating cir- A slgnal modulating yst m comprising an ouits of Figs 1 3 and 4, other circuits can be input circuit and an output circuit, means for devised where an electronically variable capaciapplymg a Carrie! Signal to d nput circuit, tance is utilized for coupling an input circuit to a resonant Circuit f r Coupling said input circuit an output circuit for developing a carrier signal to solo output circuit a clud ng a variable in the output circuit that is amplitude modulated lumped capacltence element prising two in accordance with changes of the variable capaci- Spaced electrodes, means for p ng an electahoe tron beam and for passing it through the space While there has been described what are at between said electrodes without impacting said ,present considered the preferred embodiments of electrodes, and means for Varying the p ci the invention it will be obvious to those skilled between said electrodes and the outer elemental in the art that various changes and modifications portions of said electron beam in accordance with may be made therein without departing from the to a modulatms slgnel to vary e capacitance of invention, and it is therefore aimed in the said electrodes, thereby to develop an amplitudepended claims to cover all uch change and modulated carrier signal in said output circuit.

modifications as fall within the true spirit and A o f modulating system comprising on scope of the invention lnput clrcuit and an output c rcuit, means for What is claimed is: applying a carrier signal to said input circuit,

1. A signal modulating system comprising an means coupledto said input and output circuits input circuit and an output circuit, means for and mcludme vanable element lh y applying a carrier signal to Said input circuit lumped capacitive reactance and comprlslng two means including a variable capacitance element Spaced electrodes mean? for developing a 6160- for coupling said input circuit to said output cirtron beam and for Passmg it t rough the space cuit, said variable capacitance element including between Said electrodes Without impacting Said two spaced electrodes, means for developing an electrodes, and means r yingthe positions of electron beam and for passing it between said elemental portions of said electron beam relative electrodes, and means for deflecting said electron to said electrodes in accordance with a, modulatbeam in accordance with a modulating signal to inc Signal to vary the capacitance of said electrodes, thereby to develop an ampHtude-modu- Number lated carrier signal in said output circmt. 1,844,941 'JOSEPH C. FERGUSON. 2,223,058 ALBERT T. MAYLE, JR. 2,241,976 5 2,243,829 REFERENCES CITED 2,270,777 The following references are'of record in the file of this patent: V ,3

UNITED STATES PATEN'IS m Number Name Date Number 1,650,934 Chubb Nov. 29, 1927 370,967

Name Date Boehm Feb. 16, 1932 Christ NOV. 26, 1940 Blewett et a1 May 13, 1941 Brett et a1 June 3, 1941 Von Baeyer Jan. 20,1942 Antalek May 9, 1944 Antalek Apr. 3, 1945 FOREIGN PATENTS Country Date Great Britain Apr. 11, 1932 

